As known in the art, nuclear magnetic resonance (NMR) logging methods provide a rapid non-destructive determination of porosity, movable fluid, permeability of rock formation and other parameters of interest. At least in part the wide-spread use of NMR logging is due to the fact that the measurements are environmentally safe and are unaffected by variations in the matrix mineralogy.
NMR logging is based on the observation that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter T.sub.1, known as the spin-lattice relaxation time. Another related and frequently used NMR logging parameter is the spin-spin relaxation time constant T.sub.2 (also known as transverse relaxation time) which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool. In addition to T.sub.1, and T.sub.2, NMR logging tools are capable of measuring the fluid self-diffusion coefficient, a parameter which refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. NMR measurements of the diffusion and the relaxation times provide information about the formation porosity, the composition and quantity of the formation fluid, and its viscosity, which are all parameters of considerable importance in borehole surveys.
NMR borehole measurements can be done using, for example, the centralized MRIL.RTM. tool made by NUMAR, a Halliburton company, and the sidewall CMR tool made by Schlumberger. The MRIL.RTM. tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications including: "Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination," by Miller, Paltiel, Millen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sep. 23-26, 1990; "Improved Log Quality With a Dual-Frequency Pulsed NMR Tool," by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sep. 25-28, 1994). Details of the structure and the use of the MRIL.RTM. tool are also discussed in U.S. pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200 and 5,696,448, all of which are commonly owned by the assignee of the present invention. The Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 5,055,787 and 5,055,788 to Kleinberg et al. and further in "Novel NMR Apparatus for Investigating an External Sample," by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992. The content of the above patents and publications is hereby expressly incorporated by reference.
Both centralized and sidewall tools measure relaxation times and diffusion parameters of the formation within a certain volume, i.e., the "sensitive volume" of the tool, which is determined mainly by the shape of the magnetic field generated by the tool. Specifically, the boundaries of the sensitive volume of the tool are determined by the radiation patterns of the transmitting and receiving antennae as well as the structure of the magnetic field within the receiver's frequency passband.
Centralized tools, such as Numar's MRIL.RTM. tool, have an azimuthally uniform magnetic field with respect to the axis of the borehole. Thus, the sensitive volume of the tool is a cylindrical sheet residing in the formation surrounding the borehole. Changing certain measurement parameters of the tool, such as the operating frequency, causes the diameter of this cylinder to expand or contract. In general, such tools are designed to run centralized in the borehole. By contrast, sidewall tools measure NMR signals from only one direction, and therefore operate best when the sensitive part of the tool is close to the wall of the borehole.
Centralized NMR tools have a number of advantages but typically operate best in environments having moderate conductivity. However, their operation in a highly conductive environment, e.g., in salt saturated brine, is sometimes less than optimal. Highly conductive environments typically exist when the rock itself contains salt water. In such cases, to avoid an osmotic effect which may cause the rock to collapse, the borehole fluid must further be salt-saturated. The resulting high borehole conductivity reduces the signal-to-noise ratio of the NMR measurements, thus necessitating slow logging speeds. In addition, because of the strong signal attenuation, higher peak pulse power is required, which in turn results in the need for rapid discharges of the energy storing capacitors of the tool, and also results in shortened pulse echo trains.
Furthermore, the performance of a centralized tool in highly conductive environments is decreased because of an unwanted sodium resonance in the measured signal. Specifically, the tool's gradient field causes sodium nuclei (Na-23) to resonate at about one-half of the hydrogen diameter. The amplitude of the sodium signal is about 2.25 p.u. (porosity units) per 100 kppm NaCl concentration, with a decay time between about 5 and 35 msec. For a typical 16" diameter of investigation, the sodium resonance signal is at about 8", which can create problems for measurements in boreholes having diameters of about 7.5 to 8 inches.
A standard prior art method of dealing with these problems is to reduce the fluid volume around the probe using a fluid excluder, which makes the outer diameter of the tool larger. This approach addresses adequately problems associated with signal and pulse attenuation, however, it is not satisfactory because it reduces the probe's maneuverability and does not solve the sodium resonance problem.
Accordingly, there is a perceived need for a design of a centralized NMR probe that can be used in highly conductive boreholes.